"Sovereign's Day" is traditionally held on the day of the day of the reigning Prince's patron saint. Rainier III departed from this custom by choosing the day of his enthronement, 19 November, which also happens to be the celebration of Rainier d'Arezzo. In memory of his father, Prince Albert II kept the same date, thus fixing the date of Monaco’s National Festival on 19 November..Sunday 18 November and Monday 19 November, Principality of Monaco
The current National Festival dates back to the reign of Prince Charles III, when the Principality became a modern, truly independent state with its own flag, diplomatic representation and treaties with foreign powers.
In the second year of his reign, in 1857, Charles III decided that 4 November, the day of his patron saint, would be declared the "Sovereign's Festival".
During his reign, the day that came to be known by Monegasques as the "Prince's Festival" was regularly celebrated with a Te Deum in the church of Saint-Nicolas (the cathedral whose construction began in January 1875, was only inaugurated - unfinished - in April 1884). As early as 1871, following the Te
Deum, the first military parade took place on the Place du Palais with the Prince's Guards. There were also joyous celebrations, fireworks, concerts and popular games on the Rock. It was not until 1875 that the "Carabiniers" took part in the parade.
On 19 November 1949, the accession to the throne of Prince Rainier III was celebrated, and Sovereign Order n° 465 of 27 October 1951 definitively fixed the date of the National Festival, also the Festival of Saint-Rainier, on 19 November.
H.S.H. Prince Albert II retained the date of 19 November, in preference to 15 November, Saint Albert's Day, to celebrate the National Festival.